DESCRIPTION (Adapted from applicant's description): Some persons (copers) are able to fully compensate for the absence of the anterior cruciate ligament (ACL) while others (non-copers) are not. The specific hypotheses of this proposal are: (1) non-copers reduce the force with which the foot hits the ground, and stiffen the knee via co-contraction of the muscles around the knee; (2) copers have initial contact forces that are normal, and coordinated activation of uniarticular and biarticular muscles to efficiently transfer power among the hip, knee and ankle joints in order to finely control stability of the knee joint without compromising mobility; and (3) the movement strategy that develops after ACL injury is independent of passive joint laxity and only partially dependent on quadriceps muscle strength in both non-copers and copers. The purpose of this study is to: identify differences in movement patterns (joint kinematics and kinetics and muscle activity patterns of the hip, knee and ankle) which copers and non-copers use to stabilize their knees and to compare them to normal patterns of healthy subjects during walking, jogging and curb activities; identify the movement patterns in the non-coper sample in order to characterize the mechanisms by which knee stiffening is accomplished; identify the movement patterns in the coper population in order to characterize the mechanisms by which the dynamic knee motion is preserved without compromising the knee stability; ascertain the degree to which quadriceps strength determines the stability of the copers and non-copers; and determine the degree to which moderation of contact forces contributes to the stability strategy for the copers and non-copers. The activity patterns of biarticular muscles that act at the knee and monarticular muscles at the hip, knee and ankle will be measured along with hip, knee and ankle joint kinematics and kinetics during activities which places escalating demands on the knee (walking, jogging and ascending and descending a curb) in 10 copers, 10 non-copers with quadriceps weakness, 10 non-copers without quadriceps weakness, and 10 healthy subjects. The differences between the groups as well as the relationship between quadriceps strength, initial contact force and measured gait parameters will also be assessed. The long term goal of this study is to use the differences between copers and non-copers to develop a method for evoking the compensation strategy employed by the copers in the non-copers.